The invention relates to a process for manufacturing a carbon particle filter from a porous ceramic material, which can be used to treat exhaust products of internal combustion engines and which has spaced inlet channels which open towards an exhaust gas inlet side and are separated from outlet channels which are open towards a gas outlet side.
German Patent 3,444,472 discloses a carbon particle filter whose carbon filter body consisting of porous ceramic material with a plurality of channels which extend parallel to the exhaust gas flow direction and which are alternatively sealed at the upstream or downstream end by a fitted stopper. This alternate sealing of the channels, however, is performed only on the ready-produced carbon filter body and therefore constitutes a relatively resource-intensive fabrication stop.
It is therefore the object of the invention to indicate a process for preparing a carbon particle filter from a porous ceramic material, with spaced inlet channels which open towards the exhaust gas inlet side and are apart from outlet channels which are open towards the exhaust outlet side, and without any separate alternate sealing of individual channels.
This object is achieved by having a slip-like ceramic starting material applied to a fabric having longitudinal threads extending at least in the longitudinal direction of the filter area and then layering the coated fabric to form a carbon filter body. The carbon filter body is then ceramically fired with outlet channels being produced by burning away of the longitudinal threads.
In the manufacturing process according to the invention, the position of the channels is already fixed by the arrangement of the longitudinal threads of the fabric coated with the slip-like ceramic starting material. These longitudinal threads are arranged displaced relative to one another and in line with the exhaust gas flow direction. Following the subsequent layering on top of one another or rolling up of the fabric coated with the slip-like ceramic starting material, there is present a carbon filter body in which there extend, starting from both the upstream and downstream ends, longitudinal threads which do not quite reach as far as the opposite end of the carbon filter body. During the subsequent firing process, which is used to harden the carbon filter body, the longitudinal threads, owing to their relatively low heat resistance, burn off. As a result, a carbon filter body consisting of a porous ceramic material and having the desired alternately sealed channels is obtained. Consequently, any subsequent and expensive sealing of individual channels is dispensed with.
The connection between the longitudinal threads is in the form of transverse threads which consist of a material whose burn-off temperature is above the firing temperature of the carbon filter body. This has the advantage that during the layering of the coated fabric there always remains a minimum distance between individual longitudinal threads, thereby eliminating the possibility, after these longitudinal threads have been burned away, of short-circuit formation, i.e., a connection between a channel open towards the upstream side and a channel open towards the downstream side.
There is an advantage in arranging whole layers of fabric coated with the slip-like ceramic starting material displaced relative to one another in the exhaust gas flow direction and then placing these layers in a layered arrangement. Here, not only the longitudinal threads, but also the transverse threads, can consist of one and the same material which has a low heat resistance. Such layers or mats of fabric consisting of one material are relatively inexpensive. In this case, the transverse threads are burned away as well. This is of no importance since between the individual layers of fabric, arranged displaced relative to one another, there are no connections whatsoever in the form of any transverse threads. Thus, the transverse threads in each and every instance interconnect only those longitudinal threads connected to one and the same end of the carbon filter body.
Providing the longitudinal, and optionally the transverse threads, with a rough surface has the effect that after the threads have been burned away, the individual channels likewise have an enlarged surface area and consequently ensure an improved gas transfer through the porous ceramic material. In addition, an enlarged channel surface area gives an increase in the carbon particle storage capacity.
An additional reduction in the level of pollutants in the exhaust gas, in particular hyrdrocarbon, nitrogen oxide and carbon monoxide portions, is obtained by providing the longitudinal threads with a catalytic coating.
Unlike a conventional manufacturing process for the carbon filter body by extrusion, the process according to the invention has the advantage that it is possible to use any desired "slip-capable" materials, for example aluminum dioxide or aluminum titanate.
Also, the process can utilize at least two layers of coated fabric which are placed on top of one another displaced relative to one another in the exhaust gas flow direction and then wound up to form a carbon filter body. Here, prior to the winding operation, a protruding end of a fabric layer is rolled or laid over in the direction of another fabric layer in such a way that this end comes to lie in a plane with the other fabric layer and at an interval therefrom. Likewise, the protruding end of the other fabric layer is rolled or laid over the first fabric layer in such a way that this latter end comes to lie in a plane with the first fabric layer and is at an interval therefrom. The space defined by the intervals is then filled with the ceramic starting material.
Alternative to the folding over process, one can place an additional fabric layer where the rolled over end would lie and have it be substantially flush with the end of an adjacent fabric layer and have it coated with slip-like ceramic starting material. This multilayered, four-piece composite is then rolled up prior to the firing.
Another alternative for at least two fabric layers guided on top of one another without any off-set therebetween is to remove a fabric strip at the end of each layer and perpendicularly to the exhaust gas flow direction. One fabric strip is removed in the area of the upstream end of one of the fabric layers, the other fabric strip is removed in the area of the downstream end of the other fabric layer. Subsequently all the fabric layers are coated with the slip-like ceramic starting material and wound up to form a carbon filter body.
One can provide different directions for flow through the filter by placing each layer at right angles to the layer below it.
The fabric can be formed by a mesh of longitudinal threads and transverse threads which extends transversely thereto to keep the longitudinal threads spaced apart. The longitudinal threads are made from a material whose burn-off temperature is equal to or below the firing temperature of the carbon filter body while the transverse threads consist of a material whose burn-off temperature is above this firing temperature.
When the filter is formed from a multilayered composite, the longitudinal and transverse threads can both be made of a material whose burn-off temperature is equal to or below the firing temperature of the carbon filter body.
The longitudinal threads can be located in a film or matting which dissolves during the process of layering. Also, the longitudinal threads can be sheathed with a gas-permeable material which acts as a spacer and the burn-off temperature of which is above the firing temperature of the carbon filter body. The area of the channels can be increased by having the longitudinal threads be made wavy. The longitudinal threads can consist of organic fibers or of man-made fibers.
It is advantageous if the fabric is formed by a network of transverse threads and longitudinal thread loops. The loops should alternatively start from the inlet and the outlet sides and reach up to at most the penultimate transverse thread.
The longitudinal threads and alternately the transverse threads have roughened surfaces. To ensure the performance of the filter, the longitudinal threads can be catalytically coated. Also, the cross-section of the channels open toward the upstream side can be made greater than the cross-section of the channels open towards the downstream side.
The fabric can be made as a fleece and the fleece can be perforated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.